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Friday, June 29, 2012

Prospecting in Gabbro

Gabbro a dark colored mafic rock
Photo by Tanno4595

Gabbro deposits are noted for their polymetallic sulfides that are often associated with the base of the intrusion in the form of Volcanogenic Massive Sulfides caused by settling of the sulfides to the bottom of the intrusion.  They are usually found on the footwall of the intrusion where it meets with a different kind of rock that was intruded. This is assuming that the intrusion was injected in one shot as many of them are.

If the intrusion underwent more then one injection of magma you are more likely to have a series of bedded ore bodies that are at the bottom of each individual injection as a result of segregation of the components of the magma.  These are primary ore deposits derived from the magma itself.

The ores most apt to be found associated with a Gabbro intrusion aside from iron pyrite and phyrottite are the sulfides of nickel and copper with accessory mineralization of chromium, silver, gold and platinum group metals (PGM).  More often then not these are segregated into the volcanogenic massive sulfides (VMS) at the bottom of the intrusion, or if the intrusion cooled quickly dispersed throughout the body of the intrusion. 

Some good examples of Gabbro intrusions exist on opposite ends of Connecticut.  The first intrusions of interest are located in Torrington and Litchfield they are the Hodges Complex in Torrington, and Prospect Hill in Litchfield.  Both of these prospects have had producing copper-nickel mines located on them during the early 1800s.  They were also explored for nickel after WW II.  Hodges Complex by Falconbridge mining from Canada and Prospect Hill by Dino Testone a local prospector who owned a diamond drilling rig.

In the case of the Hodges Complex not enough ore was found to make a modern mine.  The Prospect Hill Complex was another story, the diamond drill turned up a large quantity of nickel ore, but Mr. Testone passed on before he could develop a mine as told to the author by Mr. Testone.

At a later time with another geologist the author did a bedrock mapping project on the Hodges Complex and found it to be divided into three lobes that they were norite interbeded with an untramafic rock called pyroxenite.  We found several minerals in the intrusion including: ilmanite, sperrylite, erytherite, chalcopyrite, cordierite, pyroxene, labradorite, plagioclase, serpentine, and pyrite.

Both of these intrusions were diatremes that from other like diatremes were injected as wine glass shaped intrusions.  Most of the separated minerals were out of sight, and probably at the bottom of the intrusions, or in their footwalls.

The other two intrusions are located in Eastern Connecticut.  The first one is the Preston Gabbro located in the towns of Preston and Ledyard.  Most of these intrusions are found on the lands of the Mashentucket Pequot’s and on the grounds of their casino “Foxwoods.”   According to their former chief Richard Hayward in a telephone conversation several years he told the author that they found platinum in the drill cuttings when they were drilling wells for Foxwoods.  There is a platinum nugget on display in the Wesleyan University Science Museum that came from the river draining this intrusion.

There is another intrusion of Gabbro to the Northwest just below Willimantic that may be a larger part of the Preston Gabbro that could be connected below the younger rocks that form its roof.  It will take some drilling however to prove this however.

To prospect in either of these intrusions probably the best place to look for mineralization is in their footwalls where any sulfide minerals might have collected.  This may be done looking for magnetic anomalies or by core drilling.

The State of Connecticut maintains records of wells that have been drilled in the state at DEP Headquarters, 79 Elm St., Hartford, CT.  This is a good place to look because this is the office of the State Geologist as well as the Reading Room for State Environmental Records.  It also houses the DEP Store that sells State Publications.

Another nearby source of information is the State Library on Capitol Ave. across the street from the State Capitol.  They keep aerial photos reaching back to 1934 as well act as a government repository for documents published by the US Government.


General Types of Auriferous Deposits, Alluvial Exploration and Mining,

Bedrock Geological Map of Connecticut, Compiled by John Rodgers, 1985,

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